This invention relates to a method of producing zirconia powders by the forced hydrolysis of zirconyl chloride and more particularly to a method for producing powders having a uniform sub-micron particle size.
Finely divided powders of zirconia or mixtures of zirconia with one of its stabilizing oxides are useful in the preparation of ceramic or refractory articles having such diverse applications as catalyst supports, filters, extrusion dies or nozzles, protective linings, etc. Particularly useful in the general production of ceramic articles are powders which are highly dense and substantially spherical and which have a relatively uniform, sub-micron, particle size. Ceramic materials, and particularly zirconia powders, having these characteristics tend to sinter at lower temperatures, saving time and energy in the production of ceramic articles based thereon, and can provide greater strength and structural integrity to those articles. In ceramic production, the ceramic powders are normally molded or pressed into a desired shape, the so-called "green" shape, or are tape cast, followed by sintering at elevated temperatures to fuse the powders with the purpose of producing a coherent and strong body. Pressing and sintering of dense spherical particles of substantially uniform sub-micron size is desirable to provide the needed strength to the articles. If low-density particles, those having internal pores or voids, are used, excessive shrinkage of the material can occur during sintering, which can reduce the strength and increase the likelihood of cracking in the final article. Use of irregularly-shaped or large particles tends to leave larger void spaces after packing or pressing in the green state, which can weaken the final ceramic body and increase its susceptibility to cracking. Uniformity of particle size is also desirable to prevent localized exaggerated grain growth, which can result during sintering when the particle size distribution is not narrow. A typically large grains, which can grow as a result, can cause flaws that adversely effect strength.
The ability to form zirconia powders having these particle characteristics, without the need to mill or grind the material, is also commercially important. Grinding and milling often provide irregularly-shaped particles, and not only are expensive but also unavoidably introduce impurities into the powder.
It has already been shown that the production of sub-micron or colloidal mono-sized particles of various hydrated metal oxides can be obtained by forced hydrolysis. See "Monodispersed Metal (Hydrous) Oxides", E. Matijevic, Acc. Chem. Res., Vol. 14, 22-29 (1981). In that article, it is shown that maintaining acidified solutions of metal salts at elevated temperatures for definite periods of time can produce the desired metal oxide particles. It is there disclosed that the preparation of uniform particles depends upon the controlled generation of precipitating solute so that only a single "burst" of nuclei occurs and that further solute formation does not manifest itself in secondary nucleation but rather in diffusion onto the particles existing from the original nucleation. The original nuclei, accordingly, grow uniformly to yield monodispersed systems. The use of forced hydrolysis is difficult, however, because the conditions of salt concentration, pH, anion nature, temperature, etc. that are required for this controlled solute generation lie in a very narrow range. Although the discovery of the correct parameters has permitted application of forced hydrolysis in the production of monodispersed sols of, for example, alumina and hematite (iron oxide), the production of monodispersed zirconia through this procedure has not heretofore been attained.